1. Field of the Invention
The present invention relates to a source of extreme ultraviolet (EUV) radiation. More particularly, the present invention relates to an electrode-less gas discharge device in which plasma is confined in a magnetic mirror and made to radiate by resonant magnetic compression.
2. Background
As the feature size of semiconductor devices continues to decrease, the wavelength of the light utilized in the lithographic process must also decrease accordingly. Recent developments in the semiconductor arts have created the need for a source of extreme ultraviolet (EV) light of wavelength around 13.45 nm. For example, some of the required source parameters are described in the patent by R. Bristol, “EUV source box,” U.S. Pat. No. 6,809,327, Oct. 26, 2004.
Prior art methods of generating 13.45-nm EUV have included laser-produced-plasma sources and electrode-driven gas discharges. For example, the following patents disclose laser-produced plasmas: U.S. Pat. No. 6,304,630 to Bisschops, et al.; U.S. Pat. No. 6,007,963 to Felter, et al.; U.S. Pat. No. 6,469,310 to Fiedorowicz, et al.; U.S. Pat. No. 6,760,406 to Hertz, et al.; U.S. Pat. No. 6,912,267 to Orsini, et al.; U.S. Pat. No. 6,865,255 to Richardson.
Likewise, the following patents disclose electrode-driven gas discharges: U.S. Pat. No. 6,894,298 to Ahmad, et al.; U.S. Pat. No. 4,994,715 to Asmus, et al.; U.S. Pat. No. 5,335,238 to Bahns; U.S. Pat. No. 6,703,771 to Becker, et al.; U.S. Pat. No. 6,172,324 to Birx; U.S. Pat. No. 4,504,964 to Cartz, et al.; U.S. Pat. No. 6,356,618 to Fornaciari, et al.; U.S. Pat. No. 6,677,600 to Ikeuchi; U.S. Pat. No. 6,815,700 to Melnychuk, et al.; U.S. Pat. No. 6,788,763 to Neff, et al.; U.S. Pat. No. 6,167,065 to Rocca; U.S. Pat. No. 6,804,327 to Schriever, et al.; U.S. Pat. No. 6,576,917 to Silfvast; U.S. Pat. No. 6,498,832 to Spence, et al.; U.S. Pat. No. 5,317,574 to Wang; U.S. Pat. No. 6,026,099 to Young.
Laser systems have drawbacks including a high power requirement and a high cost of ownership. Gas discharges, on the other hand, are inexpensive and efficient. However, electrode-driven gas discharges will not likely meet the requirements of long lifetime, clean (essentially debris-free) operation, and stability. As is known in the art, electrodes are eroded by adjacent plasma, creating debris and limiting lifetime. Furthermore, parallel currents yield an unstable plasma-magnetic-field geometry, limiting reproducibility.
The present invention overcomes the disadvantages and limitations of the prior art by efficiently assembling a hot, dense, uniform, axially stable plasma column with magnetic pressure and inductive current drive. It employs theta-pinch-type compression of plasma confined in a magnetic mirror. The following patents disclose related prior art: I. O. Bohachevsky, “Beam heated linear theta-pinch device for producing hot plasmas,” U.S. Pat. No. 4,277,305, Jul. 7, 1981. In this and other linear theta-pinches, the plasma is heated by magnetic compression, but it is not confined axially, nor prevented from impacting its cylindrical container when the magnetic field drops. K. Fowler, et al., “Plasma confinement apparatus using solenoidal and mirror coils,” U.S. Pat. No. 4,166,760, Sep. 4, 1979. In this and other mirror machines, magnetic mirrors are used to confine electrons and ions at low densities, in large volumes. There is no buffer plasma to isolate the wall, nor unequal mirror strengths to make plasma flow to a debris dump. R. M. Hruda, “Electrodeless discharge adaptor system,” U.S. Pat. No. 3,950,670, Apr. 13, 1976. In this and other electrode-less plasma discharges, high frequency changing magnetic fields induce curling electric fields that ionize gas and drive currents. However, the plasma is not magnetically confined, nor heated by magnetic compression, nor made to magneto-acoustically resonate with the driving field.
In addition, preferred embodiments of the present invention would utilize specialized materials and auxiliary systems, such as are disclosed, for example, in the following patents: B. J. Rice, et al. “Electrical discharge gas plasma EUV source insulator components,” U.S. Pat. No. 6,847,044, Jan. 25, 2005; N. Wester, “Thermionic-cathode for pre-ionization of an extreme ultraviolet (EUV) source supply,” U.S. Pat. No. 6,885,015, Apr. 26, 2005.